The invention relates to an amplitude-transforming ultrasonic sonotrode which may be excited by way of an active, longitudinally functioning ultrasonic system. The invention further relates to an ultrasonic means which contains an ultrasonic sonotrode according to the invention.
In ultrasound technology which covers many fields, in particular in laboratory and processing technology, so-called ultrasonic disintegrators or ultrasonic homogenisators have been applied for decades, with chiefly longitudinally functioning, rod-like ultrasonic sonotrodes which are driven via an active ultrasonic system, function according to the longitudinal oscillation mode, mostly have a length n×λ/2 and apply a reproducible ultrasonic power via their end-faces.
This tried and tested technology permits the introduction of very high ultrasonic amplitudes into fluids or pasty media via the end-face of the sonotrodes, often with the help of so-called amplitude transformers (boosters). As rule, the amplitude transformation is effected at the cost of a reduction of the cross-section of the mostly circular end-face or irradiation surface. If one assumes that the transverse dimensions—for example the sonotrode diameter—usually lies below λ/4-material wavelength, one may assume approximately equal ultrasonic amplitudes (deflection and phase) at the end-face of the sonotrode. This ensures an exact and reproducible operation, for example in analysis, since the deflection of the sonotrode is always effected in a very uniform manner over the whole end-face, has approximately equal amplitude values and effects caused by the ultrasound may be linked to this parameter.
As a rule, sonotrodes and tips of such sonotrodes consist of very oscillation resistant and simultaneously low-loss materials such as titanium for example. Special ceramic or glass materials are also applied.
Micro-pipetting or deep-well plates play an increasing role in the analysis and the preparation of samples. With the acoustic irradiation of these very small sample volumes, called wells, the acoustic irradiation effort for micro-pipetting plates with 96 or more wells for example is very high with a single and necessarily thin sonotrode tip.
Various ideas for solutions have been made to overcome this exemplary problem. On the one hand one attempts to acoustically irradiate a complete micro-pipetting plate indirectly with ultrasound, from below via the base with ultrasound. For this, the plate is applied into a shallow ultrasound bath, wherein the bath consists of a turned groove or recess and of a very thick sonotrode at the end-face, which for this purpose is operated upside down. An active ultrasonic transducer which is firmly coupled to the sonotrode and which is fed by an HF generator serves as an ultrasonic source. However the influence of a transverse contraction increases significantly with an increasing diameter or thickness of sonotrodes, since the diameter of the sound conductors to the quarter wavelength in the material is larger than one already for relatively low ultrasonic frequencies. A formation of additional oscillation nodes, phase differences of the oscillation and amplitude distortions occur at the end-face of these sonotrodes. A very irregular and non-reproducible sound irradiation of the wells in the micro-pipetting plate via the large bath or irradiation surface is the result of this.
A further solution possibility may be derived from the very wide and simultaneously narrow sonotrodes which are often used in ultrasonic welding technology.
Such special sonotrodes are most usually also driven via an active ultrasonic system. They however have the disadvantage that their wide and simultaneously narrow end-face does not uniformly irradiate the high ultrasonic amplitudes due to the coupled longitudinal and bending oscillation mode. Zones with a greater and weaker amplitude alternate along the end-face. The simultaneous and direct sound irradiation of several wells of a micro-pipetting plate in principle would be able to be carried out via the distanced placing of smaller and thinner tips in a row on this surface (end-face). However one may not achieve any identical sound irradiation results in the wells of micro-pipetting plates on account of the previously mentioned differences with the deflection amplitudes.
A further solution possibility for the sound irradiation of the smallest of sample quantities in micro-pipetting plates is described in DE 101 48 916 A1.
The core and simultaneously disadvantage of the arrangement described there is the fact that the width of the emitting location of the sound waves does not exceed the width or the diameter of the active, driven ultrasonic system. On account of this, one already requires several transducers with wave-transmitting intermediate elements up to the emitting location for the sound irradiation of only one row of a micro-pipetting plate. For this reason already two units of the arrangement described there are required merely for the short side of a micro-pipetting plate. Added to this is the effort and expense for the exact positioning and mounting of the units with respect to the plate as well as the increased electronic activation effort for two or more active ultrasonic systems by the HF-generator.